The objective of this program is to develop a research resource for studying the role of trace elements in cellular processes by localizing the elements within the organelles of the cell using micron scale computed tomography (CT) of synchrotron induced fluorescent X-rays. Because fluorescence CT needs only minimally prepared, thick samples, the probability of contamination should be much lower than for techniques which require the physical sectioning of the specimen. Fluorescence CT should retain the high sensitivity of Synchrotron Radiation Induced X-ray Emission. To determine feasibility, the beam of the X-ray microprobe (1) at the National Synchrotron Light Source will be collimated to a 10 micron spot. Samples will be translated and rotated (2) through the beam using the presently installed CT apparatus (3). Iron fluorescence X-rays will be detected from cultured mouse hepatocytes. Transmitted X-radiation will be detected simultaneously so that both fluorescence and absorption tomograms can be reconstructed. In addition to showing whether concentrations of iron can be mapped within organelles, this study will help to clarify the role of iron in the free radical reactions that contribute to oxygen dependent tissue injury. In phase II, a specialized apparatus for fluorescence CT with 1 or 2 micron resolution will be designed and built.